The field of the invention relates to optical refractory sensors for determining the composition of a fluid medium. In one particular aspect of the field, the invention relates to engine air/fuel ratio control based on a determination of the composition of an alcohol/gasoline fuel mixture utilizing optical reflection detection.
Demand is increasing for motor vehicles which are operable with a mixture of alcohol, such as methanol or ethanol, and a hydrocarbon fuel such as gasoline or diesel oil. Since availability of both alcohol and gasoline will vary geographically and seasonally, vehicles are needed which may operate with any fuel mixture between 15% alcohol/85% gasoline and 100% gasoline. Further, the fuel blen may vary between refueling events such that the exact blend in a fuel tank may never be known by the operator. One problem with such vehicles is that alcohol has approximately one-half the energy density of gasoline. Thus, to maintain adequate power and drivability, the fuel delivered for each combustion event must increase in relation to the alcohol content of the fuel mixture. Other engine operating parameters such as ignition timing may also be altered as a function of alcohol content. Accordingly, a need exist for highly accurate sensors to detect the constituent composition of an alcohol/gasoline fuel mixture.
Typical optical sensors which determine the amount of alcohol and gasoline combined in a fuel mixture are disclosed in U.S. Pat. No. 4,438,749 issued to Schwippert and U.S. Pat. No. 4,770,129 issued to Miyata et al. These sensors are referred to as critical angle sensors, a representative embodiment of which is illustrated herein by FIG. 1 which is labeled Prior Art. The sensor shown includes a glass column or bar having a bottom surface immersed in the fuel mixture. A light emitting diode (LED) and a photodetector are glued to opposing ends of the glass bar. For a fuel composed of 100% gasoline, a light beam (labeled as pure gasoline) is shown striking and reflected from the glass/fuel boundary (point "g") at critical angle .theta..sub.g.sup.c. The critical angle is determined by the trigonometric relationship sine .theta..sub.a.sup.c =ratio of refractive indices for gasoline and glass. Assuming a pure gasoline mixture, all light striking the boundary at an angle greater than .theta..sub.g.sup.c will be reflected. Only a portion of light striking the boundary at less than .theta..sub.g.sup.c will be reflected, the remaining portion being refracted. Similarly, for a fluid mixture of 100% alcohol, a light beam (labeled pure ethanol) is shown striking the boundary (point "a") at critical angle .theta..sub.a.sup. c and reflected therefrom. Assuming a fuel mixture of pure alcohol, all light striking the boundary at an angle greater than .theta..sub.a.sup.c will be reflected, while only a portion of light at less than .theta..sub.a.sup.c will be reflected. The photodetector is positioned such that only light striking the glass/fuel mixture boundary between points "a" and "g" is reflected onto the photodetector. Thus, an effective collection aperture of the sensor is defined by the distance between points "a" and "g" on the boundary surface.
The critical angle for a particular fuel mixture is a function of the average composition by molecular fraction of alcohol and gasoline and their respective indicies of refraction as determined by the well known Lorentz-Loreng formula. For example, the critical angle for a hypothetical fluid mixture of 50% alcohol and 50% gasoline is shown by .theta..sub.m.sup.c at boundary point "m". With this hypothetical mixture, light striking the boundary between points "m" and "g" is totally reflected to the detector. Light striking the boundary between points "m" and "a" is partially refracted into the fluid mixture and partially reflected to the detector. The unique amount of light reflected onto the photodetector is directly related to the amount of alcohol in the fluid mixture. This relationship can be derived from the Lorentz-Lorenz formula or other similar expressions. Thus, from the electrical signal generated by the photodector, the volume composition of alcohol and gasoline can be identified.
The critical angle approach using a planar interface boundary as described above has numerous disadvantages. One disadvantage is that the effective collection aperture (between points "a" and "g") on the glass/fluid boundary is relatively small resulting in a poor detected signal-to-noise ratio. Another disadvantage is that a portion of emitted light from the LED directly irradiates the detector. Compensation must be provided for this directly transmitted light or it must be blocked thereby adding complexity to the sensor. Still another disadvantage is caused by the gap between the LED and glass bar, and the gap between the detector and glass bar. These gaps are typically filed with and epoxy having a different index of refraction than glass. Accordingly, portions of light emitted by the LED are both reflected and refracted at the gap/glass boundary. A similar phenomenon occurs with reflected light at the glass/gap boundary before the detector. These factors further reduce the signal-to-noise ratio of the sensor.